Much of the work on oxygen (O2)-binding transition metal complexes has been guided by attempts to mimic biological oxygen carriers. See Niederhoffer et al., Thermodynamics of Oxygen Binding in Natural and Synthetic Dioxygen Complexes, Chemical Reviews 84 137-203 (1984); Norman et al., Reversible Complexes For the Recovery of Dioxygen, in Oxygen Complexes and Oxygen Activation by Transition Metals, Martell and Sawyer Eds. Plenum Press: New York 107-125 (1988); Jones et al., Synthetic Oxygen Carriers Related to Biological Systems, Chemical Reviews 79 139-179 (1979); Li et al., Separation of Oxygen from Air Using Coordination Complexes: A Review, Industrial Eng. Chem. Res. 33 755-783 (1994). Cobaltous bis(salicylaldehyde)ethylenediamine (“Co(salen)”) and its derivatives are one class of complexes that have been extensively studied for their ability to reversibly bind oxygen. See Pfeiffer et al., Tricyclic ortho-condensed partial valence rings, Justus Liebigs Ann. Chem. 503 84-130 (1933); Tsumaki, Coordinate valency rings. IV. Some inner complex salts of hydroxyaldimines, Bulletin of the Chemical Society of Japan 13 252-260 (1938); Bailes et al., The Oxygen-Carrying Synthetic Chelate Compounds. VII. Preparation, Journal of the Amer. Chem. Soc. 69 1886-1893 (1947). Co(salen) is the first reported synthetic reversible Co(II) oxygen carrier to bind oxygen in the solid state, and it is believed that the O2 adduct consists of dimeric [Co(salen)]2O2 units. See Suzuki et al., Resonance Raman Spectra, Excitation Profiles, and Infrared Spectra of [Co(salen)2]O2 in the Solid State, Inorg. Chem. 20 1993-1996 (1981); Hester et al., Resonance Raman Studies of Transition Metal Peroxo Complexes: 5-The Oxygen Carrier Cobalt(II)-salen and its μ-Peroxo Complexes, [L(salen)Co]2O2: L=DMSO, py, DMF, pyO, and no L, Journal of Raman Spectroscopy 11 49-58 (1981). However, there are several polymorphs of Co(salen), with only a few having the ability to bind oxygen. See Suzuki et al., Resonance Raman Spectra, Excitation Profiles, and Infrared Spectra of [Co(salen)2]O2 in the Solid State, Inorg. Chem. 20 1993-1996 (1981); Delasi et al., The Crystal Structure of the Oxygen-Inactive Form of Bis(salicyladehyde)ethylenediiminecobalt(II), Inorg. Chem. 10 1498-1500 (1971). Formation of the different polymorphs is dependent on the preparative method, and heating and grinding can lead to interconversion between O2 active and inactive forms. See Suzuki et al., Resonance Raman Spectra, Excitation Profiles, and Infrared Spectra of [Co(salen)2]O2 in the Solid State, Inorg. Chem. 20 1993-1996 (1981). Furthermore, a significant increase in oxygen affinity occurs when an additional ligand is coordinated to the Co(salen) complexes; for example, the formation of five-coordinate square pyramidal complexes promotes oxygen binding. See Li et al., Separation of Oxygen from Air Using Coordination Complexes: A Review, Industrial Eng. Chem. Res. 33 755-783 (1994); Sharma et al., Design, Synthesis, and Characterization of Templated Metal Sites in Porous Organic Hosts: Application to Reversible Dioxygen Binding, J. Amer. Chem. Soc. 122 8946-8955 (2000).
Cobalt-O2 carriers have been studied for a variety of applications in the solid state, focusing on oxygen separation and storage. See Norman et al., Reversible Complexes For the Recovery of Dioxygen, in Oxygen Complexes and Oxygen Activation by Transition Metals, Martell and Sawyer Eds. Plenum Press: New York 107-125 (1988); Li et al., Separation of Oxygen from Air Using Coordination Complexes: A Review, Industrial Eng. Chem. Res. 33 755-783 (1994). Compared to iron complexes, such as those with porphyrin ligands, cobalt complexes have shown more potential as oxygen sorbents for air separation. See Norman et al., Reversible Complexes For the Recovery of Dioxygen, in Oxygen Complexes and Oxygen Activation by Transition Metals, Martell and Sawyer Eds. Plenum Press: New York 107-125 (1988); Li et al., Separation of Oxygen from Air Using Coordination Complexes: A Review, Industrial Eng. Chem. Res. 33 755-783 (1994); Meier et al., Tetrabutylammonium Tetracyanocobaltate(II) Dioxygen Carriers, Inorg. Chem. 36 1707-1714 (1997); Hutson et al., Synthesis and Characterization of the Sorption Properties of Oxygen-Binding Cobalt Complexes Immobilized in Nanoporous Materials, Industrial Eng. Chem. Res. 39 2252-2259 (2000). Co(salen) complexes and its derivatives have been extensively studied for this application. See Li et al., Separation of Oxygen from Air Using Coordination Complexes: A Review, Industrial Eng. Chem. Res. 33 755-783 (1994). For example, the U.S. Air Force attempted to use the compound to sequester oxygen from air but technical obstacles, including slow oxygenation rates, irreversible oxidation, and sensitivity to moisture have hampered successful utilization. See Li et al., Separation of Oxygen from Air Using Coordination Complexes: A Review, Industrial Eng. Chem. Res. 33 755-783 (1994).
Some polymorphs of Co(salen) and its derivatives have been used as homogeneous oxidation catalysts in solution. See Wei et al., CO2-Expanded Solvents: Unique and Versatile Media for Performing Homogeneous Catalytic Oxidations, J. Amer. Chem. Soc. 124 2513-2517 (2002); Musie et al., Autoxidation of Substituted Phenols Catalyzed by Cobalt Schiff Base Complexes in Supercritical Carbon Dioxide, Inorg. Chem. 40 3336-3341 (2001). Some polymorphs of Co(salen) and its derivatives have also been used as oxygen carriers (see Sharma et al., Design, Synthesis, and Characterization of Templated Metal Sites in Porous Organic Hosts: Application to Reversible Dioxygen Binding, J. Amer. Chem. Soc. 122 8946-8955 (2000)) and as catalysts in porous polymeric hosts (see Sharma et al., Immobilized metal complexes in porous hosts: catalytic oxidation of substituted phenols in CO2 media, Green Chemistry 8 972-977 (2006) and Welbes et al., Confinement of Metal Complexes within Porous Hosts: Development of Functional Materials for Gas Binding and Catalysis, Accounts of Chem. Res. 38 765-74 (2005)). The complexes are sequestered in templated polymers to isolate the metal centers, thereby avoiding dimerization and improving the stability of the complex. See Sharma et al., Design, Synthesis, and Characterization of Templated Metal Sites in Porous Organic Hosts: Application to Reversible Dioxygen Binding, J. Amer. Chem. Soc. 122 8946-8955 (2000); Hutson et al., Synthesis and Characterization of the Sorption Properties of Oxygen-Binding Cobalt Complexes Immobilized in Nanoporous Materials, Industrial Eng. Chem. Res. 39 2252-2259 (2000). More recently, nanoparticles of transition metal complexes, including Co(salen), were prepared using precipitation with compressed antisolvent (“PCA”) technology. In general, nanoparticles of Co(salen) were precipitated from methylene chloride solution using supercritical carbon dioxide as an antisolvent. See Johnson et al., Nanoparticulate Metal Complexes Prepared with Compressed Carbon Dioxide: Correlation of Particle Morphology with Precursor Structure, J. Amer. Chem. Soc. 127 9698-9699 (2005); Subramaniam et al., U.S. Patent Application No. 2007/0134338, which are incorporated by reference. This was the first time that nanoparticles of such complexes have been prepared. In addition, the ability of such complexes to absorb oxygen was not known or investigated.